Method of sealing an electron tube

ABSTRACT

Method of sealing an electron tube, which is a storage tube for example, having an evaporated alkali-halide layer formed fast on the inner face of the back plate of the tube. The envelope of the tube is previously divided into two pieces so as to facilitate the evaporation of the alkali-halide crystals and thus must be sealed upon completion of the evaporation. The envelope is sealed by application thereto of heat with another heat simultaneously applied to the alkali-halide layer, thus prohibiting the layer from becoming wet.

United States Patent u 13,5s2,175

[72] lnventor Yoshihiro Uno 1 [56] References Cited Kadoma P UNITED STATES PATENTS P 830439 2,116,129 5/1938 Stringer 65 112ux {22] Flled June 4,1

2,304,714 12 1942 Strmger 3 .v 65/28X [45] tented 1971 3 255 003 6/1966 Hays 29/2517X [731 Assgnee mm? Elem Company 3,381,347 5/1968 Reinwall 29/2519X Kadoma, Osaka, Japan Primary Examiner-John F. Campbell [32] Priority June 7, 1968 Assistant Examiner-Richard Bernard Lazarus [33] Japan Att0rney.lohn Lezdey 31 43-39788 [54] METHOD OF SEALING AN ELECTRON TUBE 3 Claims, 4 Drawing Figs.

ABSTRACT: Method of sealing an electron tube, which is a storage tube for example, having an evaporated alkalihalide layer formed fast on the inner face of the back plate of the tube. The envelope of the tube is previously divided into two pieces so as to facilitate the evaporation of the alkali-halide crystals and thus must be sealed upon completion of the evaporation. The envelope is sealed by application thereto of heat with another heat simultaneously applied to the alkali-halide layer, thus prohibiting the layer from becoming wet.

PATENTEU JUN 1 I97! I I 5 N N4 3/ .9 H

INVENTQIR Y sllulmo W ATTO E METHOD OF SEALING AN ELECTRON TUBE This invention is concerned with method of fabricating an electron tube using an evaporated alkali-halide plate and, more particularly, it relates to an improved method of sealing off the envelope of the electron tube with the alkali-halide plate kept concurrently heated at a high temperature.

Typical of the electron tubes using an alkali-halide plate are the secondary electron multiplier tube and image intensifier tube, in which are utilized the specific characteristics of the evaporated alkali-halide which acts as a secondary-electronemissive surface with high quality. If the surface of the alkalihalide crystals is irradiated with electron beams, as is well known, an optical absorption band (F-band) which is usually referred to as an F-center is induced in the crystals. This phenomenon lends itself to the storage of optical information and is often utilized to constitute an image storage tube. In the following description, therefore, the electron tubes using an evaporated alkali-halide plate are exemplified in the imagestorage tube for simplicity of discussion, although the electron tubes of such type find a variety of other practically useful applications.

It is well known that in order to have the surfaces of the line alkali-halide crystals function effectively, it is essential that the crystals be notwetted. Most of the alkali-halide crystals are, however, extremely hygroscopic and thus difficulties are involved in applying the evaporated alkali-halide layer to a storage tube requiring a glass processing with use of a gas burner.

The drawbacks experienced in the prior art methods of forming the alkali-halide storage layer on the screen or backplate of a storage tube and the advantage of the method carrying outtheinvention over such prior art methods will be understood from the following description taken in conjunction with the accompanying drawing.

In the drawing:

FIGS. 1 to 3 are views illustrating different manners of evaporating the alkali-halide crystals and sealing off the tube envelope as heretofore put into practice; and

FIG. 4 is a view showing the manner according to this invention of sealing off the envelope of a storage tube.

A storage tube of known construction is, as shown in the drawing, constituted essentially by a glass envelope 1 enclosing therein an electron gun 2 and a screen or backplate 3 attached to the front inner face of the envelope 1. On the inner face of the backplate 3 is securely placed a storage plate 4 of alkali-halide to which are to be projected electron beams from the electron gun 2 for writing therein a desired optical information. V

In order to have the alkali-halide crystals evaporated to the backplate 3 in one of the manners presently employed, the alkali-halide crystals 4 are carried by a metallic crucible 5 facing the backplate 3 at a certain angle in the envelope 1 which is previously evacuated and hermetically sealed off. The crucible 5 is then heated by means of a high frequency bombarder coil 6 to which is applied a high frequency current of the order of 400 kilocycles so as to melt away the alkali-halide crystals 4 carried by the crucible 5. As a result, the alkali-halide crystals thus molten away are evaporated and caused to ad here to the inner face of the backplate 3. A drawback is en countered in this manner of evaporation in the difficulty of controlling the thickness distribution of the alkali-halide layer and in the inability of reasonably determining the angle at which the alkali-halide vapor is directed toward the backplate.

F IG. 2 illustrates another known method proposed to eliminate these drawbacks, the method being different from that shown in FIG. 1 simply in that the crucible can be moved from the outside of the envelope.

As shown, the crucible 5' containing therein alkali-halide crystals 4 is carried by a supporting rod 7 extending outwardly of the envelope within a tubular projection 8 which is integral with the wall of the envelope. At the other end of the supporting rod 7 is provided a magnetic member 9 whereby the rod 7 and accordingly the crucible 5' connected therewith can be moved through the guide piece 10 mounted intermediate the crucible and magnetic member. The crucible 5 thus arranged can be readily positioned centrally of the cross section of the envelope by suitably moving the rod 7 by means of the magnetic member 9 so that the alkali-halide vapor emerging from the crucible 5' is evenly adhered to the inner face of the backplate 3. The heating of the crucible is effected by means of a high frequency bombarder coil 6 similarly to the method of FIG. 1. 1

Upon completion of the formation of the alkali-halide storage plate 4, the crucible 5 is moved out of the envelope 1 into the tubular projection 8 by pulling the magnetic member 9 through manipulation ofa suitable magnet (not shown) from outside. When the crucible 5' is completely accommodated within the tubular projection 8, then the projection is sealed off on the plane indicated at P and the excess portion of the projection is cut off. The evaporation of the alkali-halide crystals can thus be carried out without leaving the crucible within the envelope in this manner of prior art.

The drawbacks inherent in the first example of the prior art methods are overcome in another known manner, an example being shown in H6. 3.

ln this third example of the method, the alkali-halide layer 4 is formed on the backplate 3 prior to the mounting of the electron gun 2 in the envelope. The electron gun 2 is then secured to the stern glass 11 by means of a supporting member 12. The stem glass 11 is mounted afterwards fixedly on the envelope 1 through glass processing which is to be conducted subsequent to the stage herein. indicated. With the stern glass 11 still kept disconnected from the envelope 1, as shown, there is formed a clearance 13 inbetween. Designated at 14 is a gas burner for heating the end portion ofthe envelope to thereby seal it off with the stem glass 11. A flow of dried air is introduced into the envelope 1 through a guide 15 and is discharged initially through the clearance 13 between the envelope 1 and stem glass 11 and, once the clearance is sealed off, through the clearance between the stem glass 11 and guide 15. Thus, the water vapor resulting from the combustion by the gas burner 14 is substantially kept from the alkali-halide layer 4.

As will be understood from the last two examples of the method of forming an evacuated storage tube with alkali-halide layer, tedious operations are inevitable either in dislocating the crucible from the interior of the envelope or in forcing dried air into the envelope in the course of sealing, admitting that certain advantages are attained from the disposal of the crucible or from keeping the alkali-halide layer from becoming wet.

All of the above-mentionedinconveniences going with the prior art methods can be avoided in the method of this invention without any complicated operation and device involved.

Although the method according to the invention is applicable to the sealing process as illustrated in reference to FIG. 3, it is to be discussed in connection with a mode of sealing which has heretofore required extremely meticulous techniques but which will provide for increased practical significance.

Turning now to FIG. 4 which sketchily illustrates a preferred example of the method carrying out the invention, the glass envelope 1 is previously split laterally into two halves la and lb with a gap 16 formed at a suitable distance, even as short as 5 cm., from the front end of the envelope 1 so as to facilitate the evaporation of the alkali-halide crystals onto the backplate 3.

Where, moreover, an envelope with enlarged bulb face, it may be cut off at a portion with considerably increased sectional area, making it possible to mount on the front inner face of the envelope a backplate with previously evaporated alkalihalide layer.

After the formation of the alkali-halide layer 4 on the backplate 3, the cut ends of the halves la and lb are hermetically sealed off by heating the particular portions. The heating of the cut ends of the halves la and 1b may be effected by means ofa gas burner 17 at the entire peripheries of the ends. Characteristic of the method according to the invention, the

alkali-halide layer 4 is kept heated while the gap between the halves la and lb is being sealed off for thereby prohibiting the layer to absorb the water vapor produced with the combustion by the gas burner 17. The means for heating the alkali-halide layer 4 for this purpose is constituted essentially by heating element 18, a holder 19 for retaining the heating element to the envelope 1 and transferring the heat to that portion of the envelope which in turn transfers the heat to the alkali-halide layer 4, and a shaft connected at one end with the heating element and at the other with a rotating means (not shown) for constantly rotating the envelope 1 relative to the gas burner 17 in the course of sealing the gap 16. The heating element 18 may preferably be so arranged as to heat the alkalihalide layer 4 at the temperature ranging from 100 C. to 400 C., although at a temperature from 50 C. to 100 C. the results will prove acceptable at an appreciable sacrifice in the performance quality.

The alkali-halide layer 4 thus being heated concurrently with the heating of the cut ends of the halves la and lb of the envelope 1, the alkali-halide layer 4 is prohibited from absorbing the moisture to such an extent that the layer becomes so wet as to no longer permit of the creation of an F-center therein. Separation of the envelope 1 into two halves, moreover, will provide for ease and accuracy of forming the alkali-halide layer on the backplate, since the material alkalihalide crystals can be located sufficiently close to the backplate.

lclaim:

l. A process of producing an electron tube having an evaporated alkali-halide layer formed on the inner face of the backplate mounted on the wall of the envelope of said tube, which process comprising dividing said envelope into at least two pieces at suitable portions therein to facilitate evaporation of alkali-halide crystals onto said inner face of the backplate, heating said suitable portions for hermetically sealing the envelope and concurrently heating that portion of the envelope which surrounds said alkali-halide layer.

2. A process according to claim 1, wherein said envelope is divided into two pieces laterally at a suitable distance from said front end portion of the envelope.

3. A process according to claim 1, wherein said front end portion of the envelope is heated at the temperature of between about C. and 400 C. 

1. A process of producing an electron tube having an evaporated alkali-halide layer formed on the inner face of the backplate mounted on the wall of the envelope of said tube, which process comprising dividing said envelope into at least two pieces at suitable portions therein to facilitate evaporation of alkalihalide crystals onto said inner face of the backplate, heating said suitable portions for hermetically sealing the envelope and concurrently heating that portion of the envelope which surrounds said alkali-halide layer.
 2. A process according to claim 1, wherein said envelope is divided into two pieces laterally at a suitable distance from said front end portion of the envelope.
 3. A procEss according to claim 1, wherein said front end portion of the envelope is heated at the temperature of between about 100* C. and 400* C. 